1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has been conventionally employed to fabricate micropatterned semiconductor devices such as a semiconductor memory and a logic circuit using photolithography. The projection exposure apparatus projects and transfers a pattern (circuit pattern) formed on a reticle (mask) onto a substrate such as a wafer using a projection optical system.
To keep up with advances in micropatterning (an increase in packing density) of semiconductor devices, the projection exposure apparatus is required to transfer the pattern of a reticle onto a wafer with a higher resolution. A minimum feature size (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of light for use in exposure (exposure light) and is inversely proportional to the numerical aperture (NA) of the projection optical system. According to this principle, the shorter the wavelength of the exposure light, the better the resolution. In view of this, currently, a KrF excimer laser beam (wavelength: about 248 nm) or an ArF excimer laser beam (wavelength: about 193 nm) is being used as the exposure light. At the same time, the practical application of an F2 laser beam (wavelength: about 197 nm) and extreme ultraviolet (EUV) light is in progress.
The projection exposure apparatus is also required to expose a wider exposure region. To meet this requirement, an exposure apparatus (scanner) which relatively scans a reticle and a wafer to expose a rectangular slit-like exposure region has become the current mainstream in place of an exposure apparatus (stepper) which reduces and projects a nearly square-shaped exposure region on a reticle onto a wafer and performs full-plate exposure of the wafer.
Exposure using such an exposure apparatus requires alignment between a reticle and a wafer, so the exposure apparatus includes a plurality of alignment optical systems. The alignment optical systems include, for example, an off-axis alignment optical system and TTL (Through The Lens) alignment optical system. The off-axis alignment optical system detects an alignment mark on a wafer without a projection optical system. The TTL alignment optical system is also called a TTR (Through The Reticle) optical system, and detects the position of an alignment mark on a wafer relative to that of an alignment mark on a reticle via a projection optical system.
Also, the exposure apparatus includes an oblique-incidence focus-leveling detection system as a wafer surface position detection unit. The focus-leveling detection system obliquely irradiates with light a wafer surface onto which the pattern of a reticle is to be transferred (or a reference plate surface on a wafer stage), and detects the light obliquely reflected by the wafer surface (or the reference plate surface). In the focus-leveling detection system, an image sensor which senses the light reflected by the wafer surface (or the reference plate surface) is located such that the image sensor (image sensing surface) is nearly conjugate to the reflection point of the reflected light. With this arrangement, a positional shift (defocus) of the wafer (or the reference plate) in the optical axis direction of the projection optical system is measured as that on the image sensor of the focus-leveling detection system.
When the projection optical system absorbs exposure heat or its surrounding environment changes, an error naturally occurs between the measurement origin of the focus-leveling detection system and the focal plane of the projection optical system. To combat this situation, the TTL alignment optical system also has a function of measuring such an error (focus calibration).
Alignment detection and focus detection using a TTL alignment optical system will be explained with reference to FIGS. 26 and 27.
As shown in FIG. 26, a reticle 1100 is located on the object plane side of a projection optical system 1200. A TTL alignment optical system 1300 is located to allow observation of a wafer stage 1400 (i.e., a reference plate 1500 mounted on the wafer stage 1400) through an opening 1110 in the reticle 1100. The reference plate 1500 has a grid pattern 1510 formed in it. The opening 1110 in the reticle 1100 also has a grid pattern 1112 formed in it. Also, the TTL alignment optical system 1300 includes an illumination fiber 1310, field stop 1320, half mirror 1330, imaging lens 1340, and image sensor 1350.
After the reference plate 1500 is positioned at a predetermined position, the TTL alignment optical system 1300 captures the grid pattern 1510 in the reference plate 1500 and the grid pattern 1112 in the opening 1110 in the reticle 1100 side by side, as shown in FIG. 27. Subsequently, the amount of relative shift between the grid patterns 1510 and 1112 shown in FIG. 27 in their longitudinal direction is measured, thereby performing alignment detection between the reference plate 1500 and the reticle 1100.
Focus detection between the reference plate 1500 and the reticle 1100 is done as follows.
First, the grid pattern 1112 in the opening 1110 in the reticle 1100 is captured by the image sensor 1350. The position of the reticle 1100 in the optical axis direction of the imaging lens 1340 is adjusted so as to optimize the contrast of the waveform of the captured image sensing signal. With this operation, focusing between the reticle 1100 and the image sensor 1350 (its image sensing surface) is performed.
Likewise, the grid pattern 1510 in the reference plate 1500 is captured by the image sensor 1350, and the reference plate 1500 is moved in the optical axis direction to a position based on the capturing result, thereby performing focusing between the reference plate 1500 and the image sensor 1350. With this operation, focusing among the reticle 1100, the reference plate 1500, and the image sensor 1350 (its image sensing surface) is performed.
Japanese Patent Laid-Open Nos. 62-058624, 63-220521, 04-348019, 06-324472, and 2004-266273 disclose techniques associated with the foregoing alignment detection and focus detection using a TTL alignment optical system. In addition, Japanese Patent Laid-Open No. 2002-055435 discloses a technique of focus detection by transferring an asymmetrical grid pattern on a reticle onto a wafer, and measuring the position of the formed resist pattern, instead of using a TTL alignment optical system.
In recent years, as, for example, the wavelength of the exposure light shortens, and the NA of the projection optical system increases, the depth of focus decreases extremely. Along with this trend, accuracy of aligning a wafer surface to be exposed with a best imaging plane (so-called focus accuracy) increasingly becomes stricter. Furthermore, as micropatterning of semiconductor devices advances, overlay accuracy becomes more stringent as well. Under the circumstances, to maintain a given apparatus performance, it is becoming necessary to increase the frequencies of focus and alignment calibrations. Note that the fabrication of semiconductor devices must be stopped during these calibrations. In this respect, shortening the calibration time leads to a reduction in cost (an improvement in throughput) of semiconductor devices.
Unfortunately, the techniques disclosed in Japanese Patent Laid-Open Nos. 62-058624, 04-348019, and 06-324472 obtain a position at which the waveform contrast or the light amount is maximum by scanning the wafer stage in the optical axis direction of the projection optical system, so it takes a long time to scan the wafer stage. In addition, the technique disclosed in Japanese Patent Laid-Open No. 63-220521 obviates the need to scan the wafer stage by using a decentered stop but cannot perform alignment detection concurrently with focus detection, resulting in a decrease in throughput. Japanese Patent Laid-Open No. 2004-266273 discloses alignment detection but does not disclose focus detection.
The technique disclosed in Japanese Patent Laid-Open No. 2002-055435 can detect a best focus position with high accuracy but requires the formation of a resist pattern on a wafer. Thus, this technique is inferior in throughput to the other techniques using a TTL alignment optical system.
A TTL alignment optical system in an exposure apparatus (EUV exposure apparatus) which uses EUV light as the exposure light poses problems unique to the EUV exposure apparatus. For example, first, the EUV exposure apparatus uses a reflective reticle and therefore cannot use a conventional TTL alignment optical system. Second, the EUV exposure apparatus uses multilayer film mirrors and therefore suffers from a shortage of light attributed to a low light reflectance in case of the so-called double-pass detection scheme in which the test light reciprocates a projection optical system.